The present invention relates to a recording/reproducing method of recording data at a high density in and reproducing recorded data from an optical recording medium by collecting a laser beam on the medium, and an optical disk recording/reproducing apparatus for realizing the recording/reproducing method.
In an optical recording medium such as a phase change (PC) disk and a magneto-optical (MO) disk, the medium heated is heated by narrowing down a laser beam thereon to diffraction limit so that minute marks carrying information can be recorded as change of the reflectance or the magnetization of the medium. In this case, the spot size of the laser beam obtained in diffraction limit is in proportion to the wavelength (.lambda.) of the laser beam. Therefore, it is effective to decrease the wavelength of the laser beam in order to increase the recording density.
At present, a phase change disk with a diameter of 12 cm where 650 Mbyte of data can be recorded or reproduced by using an AlGaAs semiconductor laser diode with a wavelength of 780 nm and a magneto-optical disk with a diameter of 3.5 inches where 230 Mbyte of data can be recorded or reproduced by using a semiconductor laser diode with a wavelength of 780 nm are practically used.
In order to further increase the recording capacity, a system using an AlGaInP semiconductor laser diode showing laser action at a shorter wavelength of 635 through 680 nm has been developed. A magneto-optical disk having a diameter of 3.5 inches and a recording capacity of 650 Mbyte and using a semiconductor laser diode with a wavelength of 680 nm has been first put to practical use. Furthermore, a DVD-RAM disk with a diameter of 12 cm capable of recording/reproducing 2.6 Gbytes of data by using a semiconductor laser diode with a wavelength of 650 nm is to be put to practical use. Various examinations and studies are now being made for further increasing the storage capacity to 4.7 Gbytes.
On the other hand, a semiconductor laser diode with a shorter wavelength is also under development. Continuous laser action with a blue-green laser beam at a wavelength of 510 nm obtained by using a ZnSe semiconductor laser diode and continuous laser action with a violet laser beam at a wavelength of 410 nm obtained by using a GaN semiconductor laser diode have been reported. Accordingly, if a violet laser beam can be used, 15 Gbytes of data can be recorded on a disk with a diameter of 12 cm, resulting in realizing an optical disk system capable of recording/reproducing high quality images replaceable with a VTR system.
Now, a conventional optical disk recording/reproducing apparatus will be described with reference to an accompanying drawing. FIG. 3 is a schematic diagram of an optical pickup unit of the conventional optical disk recording/reproducing apparatus. As is shown in FIG. 3, a laser beam output from a semiconductor laser diode 51 is collimated by a collimator lens 52, and the collimated laser beam is collected to diffraction limit on a data holding surface of an optical disk 54 by a collective lens 53. At this point, the temperature of a portion of the data holding surface (recording medium) where the laser beam is collected is increased, so as to cause a phase change between crystal and amorphous. In a phase change optical disk, information is recorded in accordance with a change of reflectance of the laser beam derived from this phase change. On the other hand, in a magneto-optical disk, magnetization is once erased by increasing the temperature to a Curie point or more, and a magnetic field is applied while the medium is being cooled, so as to record information in accordance with a direction of magnetization in the recording medium. In this case, information is read (reproduced) by using the Kerr effect that the polarization direction of light is changed in accordance with a direction of magnetization in a substance.
In a reproducing operation, recorded information is read by introducing a laser beam reflected by the optical disk 54 toward a photodetector 56 by a beam splitter 55.
Now, description will be given on wavelength shift where the wavelength of the laser beam output from the semiconductor laser diode 51 is shifted.
In recording data in the optical disk 54, in order to increase the temperature of the recording medium, the semiconductor laser diode 51 is required to have an output power of approximately 30 mW through 50 mW immediately after the output and approximately 10 mW through 20 mW on the optical disk 54. On the other hand, in reproducing recorded data, the necessary output power is approximately 1 mW through 3 mW, and a larger output power can destroy or erase the recorded data. Accordingly, the semiconductor laser diode 51 should conduct continuous laser action with a low output power in a reproducing operation and should conduct modulation action having a high output power and modulated by a recording signal in a recording operation. Thus, the semiconductor laser diode 51 is operated with a low output power in reproducing the optical disk 54 while it is required of a recording operation with a high output power. As a result, it is necessary to increase a driving current for the semiconductor laser diode 51 in a recording operation. Since an optical output power is increased in accordance with increase of a current at a ratio of approximately 0.5 mW/mA, a difference in the current value between a recording operation and a reproducing operation is as large as 50 mA.
On the other hand, the semiconductor laser diode 51 generally includes an optical resonator (Fabry-Perot cavity) having parallel mirror surface edges and formed in an optical semiconductor material such as AlGaAs and InGaP, so as to show laser action at a wavelength which satisfies resonating conditions of the resonator and is in the vicinity of a wavelength attaining a maximum optical gain of the material. The wavelength attaining the maximum optical gain accords with an energy gap Eg of the optical semiconductor material, but the energy gap Eg is changed in accordance with a temperature. Specifically, the energy gap Eg is decreased as the temperature is increased. Therefore, as the temperature is increased, the wavelength at which the semiconductor laser diode 51 shows laser action is increased at a ratio of approximately 0.3 nm/.degree.C.
In this manner, when the temperature of the semiconductor laser diode 51 is increased due to the current increase from a reproducing operation to a recording operation, the wavelength for showing the laser action is unavoidably increased. This phenomenon is designated as the wavelength shift (.DELTA..lambda.). The wavelength shift can be varied depending upon the ambient temperature and a heat radiation state of the semiconductor laser diode 51, and generally has an amplitude of approximately 2 nm through 3 nm.
The present inventors have found that the wavelength shift causes defocus (focal shift) of the laser beam on the optical disk 54.
The collimator lens 52 and the collective lens 53 included in the optical pickup unit shown in FIG. 3 are made from glass or plastic, and the refractive index of such a lens material is generally varied depending upon a wavelength. On the other hand, the focal length of a lens is varied depending upon the refractive index of the lens. As a result, when the wavelength is varied, the focal length is varied. This phenomenon is designated as chromatic aberration of a lens.
FIG. 4 shows an example of the change (in .mu.m/nm) of the focal length against change of a wavelength in a lens with a focal length of 3.3 mm. In general, as the wavelength is shorter, the chromatic aberration is larger. In the example shown in FIG. 4, it is understood that the chromatic aberration observed at a wavelength of 400 nm is substantially three times as large as the chromatic aberration observed at a wavelength of 650 nm.
In reproducing data from the optical disk 54, the position of the collective lens 53 is finely adjusted in the optical axis direction so that the laser beam of FIG. 3 can be always focused on the optical disk 54. This operation is designated as focus servo. The light reflected by the optical disk 54 is introduced toward the photodetector 56 by the beam splitter 55, so that the focus servo can be conducted on the basis of a signal detected by the photodetector 56. In this servo operation, since the collective lens 53 with large inertial mass is driven, a response speed of approximately 0.1 ms is necessary. However, when the wavelength of the semiconductor laser diode 51 is changed by the current increase in a recording operation and the chromatic aberration of the lens is caused, the position of the focal point of the laser beam on the optical disk 54 is changed in the optical axis direction. Accordingly, the laser beam is defocused in a recording operation, resulting in increasing the spot size. This is because the focus servo cannot follow the change since a switching time between a reproducing operation and a recording operation is as short as approximately 1 .mu.s. In this case, it is difficult to conduct high density recording.
As described so far, when the wavelength shift becomes large, defocus is ultimately caused. Therefore, the semiconductor laser diode 51 is manufactured so as to minimize the wavelength shift, and the wavelength shift actually allowed in a DVD-RAM using a semiconductor laser diode with a wavelength of 650 nm is approximately 3 nm at maximum.
However, the present inventors have found that the wavelength shift allowed at a wavelength of 400 nm is 1 nm or less. In this case, in a semiconductor laser diode including a Fabry-Perot cavity, it is very difficult to suppress the wavelength shift to be 1 nm or less even when the temperature of the semiconductor laser diode is retained at a predetermined temperature or lower.